Switching-mode power supplies generally switch at a clock frequency in the kilohertz range, with switching elements usually being activated by means of pulse-width modulated switching signals. The steep edges of the pulse-width-modulated switching signals result for example in unwanted electromagnetic interference. Such interference is dealt with in the context of the electromagnetic compatibility (EMC) of a switching-mode power supply.
A distinction is made between line-based interference and interference radiation in free space. In order to keep the impact of electromagnetic interference on other electrical devices due to unwanted coupling low, EMC regulations must be complied with. In these a frequency bandwidth is generally specified to determine average levels of the frequency spectrum of a switching signal, it not being permitted to exceed maximum permissible average interference levels. This frequency bandwidth therefore corresponds to the filter bandwidth of an interference level measuring device. Since interference at very low frequencies is generally not a problem, EMC regulations generally provide for a lower threshold frequency for the determination of average interference levels. At low switching frequencies the first harmonic of a frequency spectrum can be below this threshold frequency, so that harmonics with higher ordinal numbers are of relevance for determining the permissible interference level.
To comply with EMC regulations, complex filters for example are known from the prior art, to prevent electromagnetic interference with propagation along the lines connected to a switching-mode power supply and by way of free space.
It is also known when forming a switching signal to use frequency modulation methods to reduce average interference levels of the frequency spectrum. This attempts to spread the frequency spectrum (spread spectrum technology), resulting in a lower average interference level for the same filter bandwidth of an interference level measuring device.
Such methods generally operate with a triangular or saw-tooth modulation function, low modulation frequency and high modulation index and are already implemented in some commercially available control modules for switching-mode power supplies (e.g. topswitches). The peak values of the interference levels are not lowered in this process, as the spectral components have a lower frequency interval from one another than the prescribed filter bandwidth of an interference level measuring device.
To further improve the electromagnetic compatibility of a switching-mode power supply it is also necessary to try to reduce the interference level peak values.
The publication Wiemer I.: Freiprogrammierbare Ansteuerung eines Aufwärtswandlers (Freely programmable activation of an upward converter), Studienarbeit TU Dresden, Dresden, Nov. 30, 2001, describes a chaotic frequency modulation method using a large number of discrete modulation frequencies. It is thus possible to tailor the interference spectrum in almost any manner to the available filters and at the same time reduce the peak values of the interference levels. However this method requires the deployment of a high-powered (signal) processor.